1. Field of the Invention
The present invention relates to a mobile communication system and a communication method, and more particularly, to a reservation based mobile communication system, mobile terminal equipment, and communication method to which code division multiple access (CDMA) is applied.
2. Description of the Related Art
Conventionally, a mobile communication system which employs a reservation based access control in a frequency division multiple access (FDMA) scheme is known, for example, as described in IEEE Transactions on Communications, Packet Switching in Radio Channels: xe2x80x9cPart3-Polling and (Dynamic) Split-Channel Reservation Multiple Accessxe2x80x9d, COM-24, 8, (1976), pp. 832-845 (hereinafter called xe2x80x9cprior art publication 1xe2x80x9d).
In the reservation based access control, each of mobile terminals having a request for data transmission reserves a traffic channel to a base station through a reservation packet. The base station, after scheduling traffic channels and transmission timing (time slots) to be assigned to these mobile terminals, notifies each of the mobile terminals of transmission timing to be used on an assigned traffic channel through a reply packet. According to this reservation based access control, collision of packets on the traffic channel can be basically avoided.
As another example of reservation based control type communication system, for example, JP-A-6-311160, corresponding to U.S. patent application Ser. No. 08/230773 (hereinafter called xe2x80x9cprior art publication 2xe2x80x9d) has proposed such a communication system based on a time division multiple access scheme.
However, in the mobile communication systems in which the reservation based access control is applied to FDMA and TDMA schemes, as proposed by prior art publications 1 and 2, since respective mobile terminals send reservation packets through a reservation channel asynchronously with each other, a plurality of reservation packets can collide with a high possibility. Thus, repetitive retransmission of reservation packets obliged by the collision of packets constitutes a main cause of degrading the throughput of the entire communication system.
Meanwhile, as a standard for FPLMTS (Future Public Land Mobile Telecommunication Systems), the adoption of the code division multiple access scheme is regarded as promising. A CDMA mobile communication system has been proposed, for example, in JP-A-7-38496 corresponding to U.S. patent application Ser No. 08/375679 (hereinafter called xe2x80x9cprior art publication 3xe2x80x9d). However, prior art publication 3 does not provide any useful information for solving the problem of a degraded throughput in the reservation based access control.
It is an object of the present invention to provide a mobile communication system and a communication method which employ a reservation based access control to realize a high throughput.
It is another object of the present invention to provide CDMA mobile terminal equipment and base station which solve the problem of collision of reservation packets to realize a high throughput.
To achieve the above objects, in a mobile communication system of the present invention, radio channels include a plurality of traffic channels used for transmitting upward data packets directed from mobile terminals to a base station and for transmitting downward data packets directed from the base station to the mobile terminals, a reservation channel used for transmitting reservation packets each indicative of a traffic channel assignment request from a mobile terminal to the base station, and a reply channel used for transmitting reply packets each indicative of a traffic channel through which data is transmitted and received from the base station to a mobile terminal, wherein the reservation, reply and traffic channels are applied with spread-spectrum in accordance with a CDMA scheme. The mobile communication system is characterized in that a mobile terminal having a request for data transmission transmits a reservation packet onto the reservation channel at arbitrary timing, the base station specifies a traffic channel and a time slot to be used by the requesting mobile terminal by a reply packet transmitted through the reply channel, and each mobile terminal transmits and receives a data packet in the time slot on the traffic channel, both specified by the reply packet.
Describing in greater detail, each of the reservation, reply and traffic channels is assigned a unique spreading code, for example, pseudonoise (PN). Particularly, the reservation channel is assigned a spreading code shorter than those assigned to other reply and traffic channels. The base station relies on a matched filter to identify a plurality of reservation packet signals having time-overlapped portions, transmitted from a plurality of mobile terminals, and to perform a receiving process on bit signals corresponding to each packet.
According to a preferred embodiment of the present invention, the base station, upon receiving a reservation packet from a mobile terminal, assigns a time slot on a traffic channel in accordance with a schedule control, and notifies each mobile terminal of the assignment result through a reply packet.
Also, for regulating a total number of simultaneously communicated packets, the base station periodically transmits a busy tone signal indicative of a traffic situation, such that each mobile terminal having a request for data transmission performs a reservation packet transmission control in accordance with the busy tone signal. Alternatively, the radio channels may be provided with a plurality of reply channels so as to specify a reply channel for each mobile terminal to receive the busy tone signal therethrough.
According to the present invention, time slots are defined in the traffic channels such that each mobile terminal transmits and receives data in a particular time slot specified by the base station. The reservation channel, on the other hand, is not provided with time slots, so that each mobile terminal having a request for data transmission transmits a reservation packet at arbitrary timing, thus facilitating the operation of transmitting the reservation packet in each mobile terminal.
Also, each mobile terminal performs a spectrum spreading or multiplies the reservation packet by a spreading code to generate a spread-spectrum reservation packet, where the spreading code has a period shorter than that applied to a data packet transmitted through a traffic channel, while the base station receives reservation packets using a matched filter.
In this case, even if two or more spread-spectrum control packets, modulated by the same spreading code, are partially overlapped on the time axis, the matched filter can identify received packets, provided that there is a timing deviation over one chip or more on the spreading code between the respective packets. Therefore, even if a plurality of mobile terminals generate reservation packets individually at arbitrary timing, a reception disabled condition caused by collision of these packets will occur with an extremely low possibility.